My Mom is in need of a new car. The problem is her trade in: It is a 2002 PT Cruiser with a serious overheating problem ($1700+ quote at two reputable repair places) Now here is the problem. Do I keep my dang mouth shut when we go to the dealership and do the deal? I have a spare car that she is driving until it cools off and the overheating problem will not be noticeable at trade in.

I would never sell the car to a guy off the street without disclosing a major problem. Even to a car dealership I think I feel guilty in not disclosing it. We are not going to be financing, and will be paying cash for the car. So it is not like they can unwind the deal if they discover the problem.
Having ethical dilemma about screwing over a car dealership who exist solely to try and take as much money as they can from you in every conceivable way is weird.

Bonus question. These are the three cars we are considering Hyundai Elantra Touring, VW Jetta Wagon and Ford Focus Wagon. Any recommendations of the three or reasons to avoid them?